Catalysis of Organic..

14Supported Re Catalysts for Olefin Metathesis(as is the case for heterogeneous catalysts based on supported metal oxides and earlyhomogeneous catalysts based on mixtures of metal halide and a main groupalkylating agent), or prior to addition of the substrate (as is the case for ‘welldefined’homogeneous catalysts such as those of Grubbs’ and Schrock). A supportedorganometallic catalyst, MeReO 3 on silica-alumina, has also been reported to showactivity in olefin metathesis. In solution, MeReO 3 does not react with α-olefins, nordoes the silica-alumina support catalyze olefin metathesis. However, MeReO 3supported on silica-alumina is effective for the metathesis of both simple andfunctionalized olefins at room temperature, without further thermal or chemicalactivation (2-4).Deposition of white, air-stable MeReO 3 either by sublimation or from solutiononto calcined, dehydrated silica-alumina generates a brown, air-sensitive solid.Evidence from both EXAFS and DFT calculations suggest that Lewis acidicaluminum centers on silica-alumina represent the most favorable chemisorption sites(5). One Re=O bond is substantially elongated due to its interaction with a distortedfour-coordinate Al site. Coordination of Re to an adjacent bridging oxygen alsooccurs, creating the rigidly-bound surface organometallic fragment shown in Scheme1. Interaction with a Lewis acid is known to promote tautomerization of MeReO 3 (6),leading (at least transiently) to a carbene. However, the participation of this carbenetautomer in initiating metathesis has not been established.OHSiSiOOOAlOOReOOCH 3OSiOOOOHSiSiOSiSiScheme 1 Structure of MeReO 3 dispersed on the surface of dehydrated silicaalumina,as established by EXAFS and DFT (5).The role of SnR 4 promoters in increasing activity and conferring functionalgroup tolerance on supported perrhenate catalysts was originally suggested toinvolve in situ formation of organorhenium species such as RReO 3 (2); however,direct evidence for their participation has not been previously sought. When aperrhenate/silica-alumina catalyst is treated with SnMe 4 , methane is evolved. Thishas been interpreted as evidence for carbene formation via double methylation of